For the sake of simplicity, the projection system may hereinafter be referred to as the xe2x80x9clensxe2x80x9d; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, catadioptric systems, and charged particle optics, for example. The radiation system may also include elements operating according to any of these principles for directing, shaping or controlling the projection beam, and such elements may also be referred to below, collectively or singularly, as a xe2x80x9clensxe2x80x9d.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the mask (reticle) may contain a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto an exposure area (die) on a substrate (silicon wafer) which has been coated with a layer of photosensitive material (resist). In general, a single wafer will contain a whole network of adjacent dies, which are successively irradiated via the reticle, one at a time. In one type of lithographic projection apparatus, each die is irradiated by exposing the entire reticle pattern onto the die at once; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatusxe2x80x94which is commonly referred to as a step-and-scan apparatusxe2x80x94each die is irradiated by progressively scanning the reticle pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the substrate table (wafer table) parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally  less than 1), the speed V at which the wafer table is scanned will be a factor M times that at which the mask table (reticle table) is scanned. More information with regard to lithographic devices as here described can be gleaned from International Patent Application WO97/33205, for example.
Until very recently, lithographic apparatus contained a single mask table and a single substrate table. However, machines are now becoming available in which there are at least two independently moveable substrate tables; see, for example, the multi-stage apparatus described in International Patent Applications WO98/28665 and WO98/40791. The basic operating principle behind such multi-stage apparatus is that, while a first substrate table is at a exposure position underneath the projection system for exposure of a first substrate located on that table, a second substrate table can run to a loading position, discharge a previously exposed substrate, pick up a new substrate, perform some initial measurements on the new substrate and then stand ready to transfer the new substrate to the exposure position underneath the projection system as soon as exposure of the first substrate is completed; the cycle then repeats. In this manner it is possible to increase substantially the machine throughput, which in turn improves the cost of ownership of the machine. It should be understood that the same principle could be used with just one substrate table which is moved between exposure and measurement positions.
In a known lithography apparatus, substrates for exposurexe2x80x94such as wafersxe2x80x94can be first loaded from a wafer carrier or process track into a pre-align module, so as to prepare the substrates for exposure. One of the most important aspects of such preparation is a pre-alignment step. In this step, the wafer is placed on a pre-aligner turntable and its edge inspected by rotating it past an edge sensor, which may comprise an optical or capacitive sensor, for example. This enables automatic location of the notch or flat edge of the wafer, and also allows the eccentricity of the wafer on the turntable to be measured. In this way:
the notch or flat edge can be automatically oriented as desired, before the wafer is transferred to the substrate table;
It can be determined if the eccentricity of the wafer exceeds a threshold value which, when translated to the substrate table, would cause the wafer to fall outside the capture range of an alignment module employed at the substrate table. If such is the case, then the wafer can first be shifted on the turntable by a pre-calculated amount, so as to bring it within the threshold value.
Once these steps have been performed, a substrate handler can remove the wafer from the turntable and place it on the wafer table with a precision which will allow wafer capture by the alignment module.
However, the present inventors have determined that the various activities of the pre-aligner are a source of undesirable vibrations which can induce overlay errors if pre-alignment steps are carried out whilst another wafer is being exposed. It is undesirable to not perform pre-alignment concurrently with exposures as this reduces throughput. Also, if the pre-aligner is mechanically isolated from the wafer table to prevent transmission of vibration, their relative position will no longer be certain to a sufficient accuracy to allow the substrate handler to transfer the wafer to the wafer table with the required precision.
An object of the present invention is to provide a substrate handler capable of transferring substrates from a pre-aligner to a substrate table in a lithographic projection apparatus whilst avoiding the transmission of vibrations to the substrate table when pre-alignment steps are carried out on one substrate concurrently with exposure of another substrate.
According to the present invention there is provided a lithographic projection apparatus comprising:
a radiation system constructed and arranged to supply a projection beam of radiation;
a mask table provided with a mask holder for holding a mask;
a substrate table provided with a substrate holder for holding a substrate;
a projection system constructed and arranged to image an irradiated portion of the mask onto a target portion of the substrate;
a pre-aligner constructed and arranged to transfer out an initial alignment of a substrate; and
a substrate handler constructed and arranged to transfer a substrate from said pre-aligner to said substrate table;
said pre-aligner is mechanically isolated from said substrate table; and
coupler constructed and arranged to coupler said substrate handler to said substrate table at a known relative position.
Because the pre-aligner of the present invention is mechanically isolated from the substrate table, which carries the substrate during exposure processes, vibrations caused by the pre-alignment steps are not transmitted to the substrate during exposure. To provide positional accuracy in the transfer of substrates, the substrate handler couples to the substrate table to define its position relative thereto and only then deposits the substrate in the substrate holder. The substrate handler can have a fixed position relative to the pre-aligner or may be mechanically isolated from it as well and couple to it to pick up the substrate. Coupling to the pre-aligner ensures that, even if the substrate handler is isolated from the pre-aligner, it is at a known, e.g. predetermined, position relative to the pre-aligner when the substrate is picked up so that it will be held in the substrate handler at a known position. Similarly, when the substrate handler is coupled to the substrate table, its relative position is known and the positional accuracy of the substrate when in the pre-aligner is preserved in the transfer. If the substrate handler couples to both the substrate table and the pre-aligner, its relative position is known relative to the pre-aligner when the substrate is picked up from that pre-aligner and its relative position is known relative to the substrate table when the substrate is deposits, such that the accuracy of the pre-aligning is maintained during the transfer.
In preferred embodiments, when the substrate handler approaches the pre-aligner or substrate table their relative positions are known only coarsely. Once the substrate handler has made an initial contact with the pre-aligner or substrate table, an automatic coupling mechanism brings the two together in a predetermined physical relationship. This can be achieved using an end-effector member on the substrate handler, which is loosely coupled to the substrate handler. The end-effector member carries half of a coupling which mates with corresponding halves on the pre-aligner and substrate table. The two halves of the coupling are mechanically biased to take up the correct position on coupling, without the necessity for the substrate handler as a whole to be as accurately aligned to the substrate table.
According to a further aspect of the present invention there is provided a method of manufacturing a device comprising:
providing a mask bearing a pattern to a mask table;
providing a substrate having a radiation-sensitive layer to a pre-aligner and preparing it for exposure;
transferring said substrate from a pre-aligner to said substrate table using a substrate handler; and
imaging irradiated portions of the mask onto target portions of the substrate;
said transferring said substrate from said pre-aligner comprises:
picking up said substrate from said pre-aligner;
coupling said substrate handler to said substrate table; and
placing said substrate in said substrate holder of said substrate table.
In a manufacturing process using a lithographic projection apparatus according to the invention a pattern in a mask is imaged onto a substrate which is at least partially covered by a layer of energy-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices (dies) will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4.
Although specific reference may be made in this text to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9cexposure areaxe2x80x9d, respectively.
In the present document, the terms xe2x80x9cradiationxe2x80x9d and xe2x80x9cbeamxe2x80x9d are used to encompass all types of electromagnetic radiation or particle flux, including, but not limited to, ultraviolet radiation (e.g. at a wavelength of 365 nm, 248 nm, 193 nm, 157 nm or 126 nm), extreme ultraviolet radiation (EUV), X-rays, electrons and ions. Also herein, the invention is described using a reference system of orthogonal X, Y and Z directions and rotation about an axis parallel to the I direction is denoted Ri. Further, unless the context otherwise requires, the term xe2x80x9cverticalxe2x80x9d (Z) used herein is intended to refer to the direction normal to the substrate or mask surface, rather than implying any particular orientation of the apparatus.